Protein kinase Cs (PKC) are a ubiquitous family of regulatory enzymes that associate with membranes and are activated by diacyloglycerol or tumor promoting agonists such as phorbol esters. These lipophilic activators bind to conserved cysteine-rich domains. We have determined the crystal structure of the second cysteine-rich activator binding domain, known as 'cys2', of PXC delta by multiwavelength anomalous dispersion from native zinc ions. We have also determined the structure of the cys2 domain complex with the tumor promoting PKC activator phorbol ester. The structure explains the roles of phorbol esters and the cysteine-rich domain in the activation and membrane insertion of PKC.